I agree with this review regarding the tone, art, characters, and subsystems (time, intoxication, etc). These things alone make Witchburner worth reading.

However, the primary reason (imo) to read Witchburner over other fantasy intrigue material is the nature of the core investigation. The player characters are put into an ethically fraught situation (rather than immediate physical danger). How they handle it likely depends on how the players understand their relationship to the GM's hook. Many published 'adventures' assume that when a character in the fiction gives the party a quest ('Find the witch!'), it doubles as the GM saying 'Do this, please, it's all I have prepped.' There's a canned adventure with it's own arc, and the opportunity for a hard-won, tidy ending. The players need only participate (or even, play along). Witchburner, to its credit, does not operate on this assumption.

If the players see the adventure (find the witch) as the thread they must follow to glory and riches, they might be disappointed (or feel misled, even betrayed). Rather, this seems to assume that players will engage with a problem critically, even laterally. Do we even want to complete this quest? Do we approach it the way we're told to? Is something else going on here? These questions ratchet up a core tension, because the player characters aren't just pursuing a mystery, they have to ask themselves what they really want out of this situation.

Even though I applaud this kind of design (and sandbox games generally operate under the same assumption), it's worth noting that the module doesn't address this at all. I'm not saying it should, but people who expect to "teach" this kind of approach to adventures (from the players' perspective) won't find anything. That is to say, players used to having "the quest" might feel cheated just as you said.

Yes, some guidance around framing this adventure would be really useful. There are some proposed modifications if you think a given approach wouldn't work for your group (intentional vagueness to avoid spoilers) but no suggestions for developing group buy-in. Like a lot of OSR material it assumes you either have that skill set, or can find resources to develop it elsewhere.

I guess you sorta do that with the alternative framing of the setup (although the adventure only mentions another reason why the GM might want to use that - but I'll need to check the text again). A simple "What if my players don't question the hook?" sidebar or the like somewhere in the introduction would suffice.

I sort of assumed: "if the players don't question the hook, that's fine. Let them go through with the whole thing, none the wiser."

It's something I've been thinking about - should a module have a small essay from the author? Thoughts on how and why it was written? It's the kind of thing that I would personally quite like to see in the appendices of a game.